The U.S. State Department has ordered the evacuation of non-essential staff at the American embassy in Yemen due to terrorist threats, as reports emerge that intercepted al-Qaida communications led to the recent closure of dozens of U.S. diplomatic posts.

A statement issued Tuesday also urged U.S. citizens in the country to depart because the potential for both terrorist attacks and civil unrest is "extremely high."

CLICK TO EXPAND: Embassy, consulate closure status.

Britain has also temporarily withdrawn all staff from its embassy in Yemen due to security concerns.

Meanwhile, Yemeni security officials say a suspected U.S. drone strike has killed four alleged al-Qaida members in Marib province.

Intercepted al-Qaida message

This still image from video obtained courtesy of a group called "IntelCenter," showing Al-Qaeda leader Ayman al-Zawahiri appearing in a new video released, October 11, 2011.

Earlier, U.S. media reported that the closure of U.S. diplomatic posts was triggered by intercepted communications between al-Qaida leader Ayman al-Zawahri and the head of the terrorist group's offshoot in Yemen. The reports said that al-Zawahri ordered Nasir al-Wuhayshi, the leader of al-Qaida in the Arabian Peninsula, to carry out an attack as early as this past Sunday. Al-Wuhayshi was recently elevated by al-Zawahri as al-Qaida's second-ranked leader.

Analysts say the communications indicate that al-Zawahri is working through al-Qaida's regional affiliates now that the core group has been substantially weakened.

Some embassies were reopened Monday after a day-long shutdown, including posts in Algiers, Baghdad, Dhaka, and Kabul. Nineteen others will stay closed including Amman, Cairo and Sanaa, and Tripoli.

The State Department says it is keeping the 19 embassies closed "out of an abundance of caution." Spokeswoman Marie Harf says officials will keep analyzing intelligence as it evaluates security needs.

Nature of threat

U.S. officials have not specified the nature of the threat.

Several key U.S. lawmakers said the threats of a possible imminent attack are the most specific they have seen since the September 11, 2001 terrorist attack on the United States. They call the decision to close embassies and issue a global travel alert extraordinary. The international police organization Interpol issued its own security alert.

"This is a wake up call," warned Republican Congressman Peter King of New York. "Al-Qaida is in many ways stronger than it was before 9/11 because it's mutated and it's spread and it came out of some different directions. And al-Qaida in the Arabian Peninsula is probably the most deadly of all the al-Qaida affiliates."